In this tutorial we are going to build an amplifier circuit. We are going to accomplish this via the venerable LM386 integrated circuit (IC) operational amplifier (Op Amp). Depending on which one your electronics store has the output will be about 700–1000 mW, or about 1 Watt. The project is fun, inexpensive, quite easy if you follow the step-by-step directions, and has awesome results with which you can be impressed and do some impressing!
The way I like to start these types of prototyping project is to layout the “main” parts and the larger parts to make sure I have enough room to make connections. I also make sure to bridge the ground and power lines.
Notice, and make sure, that the LM386’s small divot is facing towards the left. This is important as it indicates the location of the 1 and 8 pins of the op amp.
It is also imperative that we take a look at the basic data sheet to find the pin assignments for the LM386 before we move forward. Any time we make a connection you should come back to this chart to get a better understanding of why.
This is a small amplifier so we can use a standard 9v battery to power it. In fact, most LM386s can take up to 12 volts. Whenever I start prototyping I like to make a simple power indicator LED circuit. What this small circuit does is lights an LED whenever power is being supplied to the breadboard, or the amp in this case. It is a good idea to do it first. That way you will always know when to board/chip is “hot”.
The circuit consists of a 220 ohm resistor going from the power rail to a column of the breadboard. Then the positive lead of the LED (the longer leg) connected to that same column and the negative lead (the shorter leg) going to a new column. The last connection, to close the circuit, is to put a jumper wire from the negative lead’s column to the ground rail.
Now, whenever power is supplied to the board the LED will light up.
Prepare the battery. What I have done here is added a switch to the positive wire of the 9v battery clip, for obvious reasons. I also added some custom jumper style tips to the ends of the wires using pieces of old leads of other components I have used in past projects. This makes it easier to connect them to the breadboard.
Once you have your battery all set up, connect the positive wire to the positive rail and do the same for the negative wire to the negative rail. Take this opportunity to turn the battery on and test your LED circuit.
Now is when the fun begins. Take a jumper wire and connect Pin 1 (gain) to the middle (signal) pin of the closest potentiometer (Pot). This will be our Gain Pot.
Now, if you remember from the pin assignment sheet of the LM386 Pin 8 is also labeled gain. We need to connect these pins through a capacitor to increase the gain factor. We will do this by first adding a 10 uF capacitor from Pin 8 to a new Column.
Now you can connect the 10 uF capacitor to the far right pin of the Gain Pot. This will increase the normal gain of 20 to a gain of 200 (20 x 10). The potentiometer will allow us to go anywhere from 20 to 200.
Also, you can make the connection to the pot’s left pin, but then you will invert the direction of the multiplier. Which means you will turn the Pot to the left to increase the value (make things louder), instead of the normal turning to the right to increase the value (increase the gain).
Now we need to connect Pin 4 to Pin 6 with a decoupling capacitor. We will use a 0.1 uF ceramic cap here.
Decoupling: make the interaction between (electrical components) so weak that there is little transfer of energy between them, especially to remove unwanted AC distortion or oscillations in circuits with a common power supply.
When bending component leads it is best to use a set of small pliers. Hold the lead using the pliers near the base of the components lead. Then bend on the opposite side. This decreases the potentially damaging strain on the component connections.
While we are dealing with this side of the IC, let's connect Pin 2 and Pin 4 to ground.
Pin 3 is going to be used for the Volume Potentiometer. What we want to do here is go from Pin 3 to a new column somewhere in between the two Pots. Then go from that column to the middle pin of the Volume Pot. While you are there, don’t forget to add the 470 pF capacitor to connect that column to the ground rail.
Connect Pin 6 to the Power Rail.
Also in this step, connect the right pin of the Volume Pot to the Ground Rail.
Add the 10k Ω resistor from Pin 7 to a new column.
We are going to get a bit crazy and add 3 capacitors in this step. One 100 uF cap decoupling cap on the power and ground power rails, then a 10 uF cap to connect the 10k resistor to the ground rail, then another 0.1 uF decoupling cap again connecting the power and ground rails.
Take a jumper from Pin 6 to a new column. Then take the 10 Ω resistor from that jumper to yet another new column.
From the column where that 10 Ω resistor ends put the monster 1,000 uF cap to a new column, this is the audio output, and the last 0.1 uF cap from that same column to the negative power rail.
Connect the alligator clips to the positive and negative connector of the speaker.
You should try to get an 8 Ω 1 Watt speaker for this project. However, I couldn’t find a 1 Watt speaker so I used a 3 Watt and it gave good results.
Connect the negative speaker connections to the negative power rail and the positive to the same column as the 1,000 uF capacitor.
Make sure your 3.55 mm jack is soldered right and ready to transmit signals.
Take the input (positive) wire of the headphone jack and connect it to the left pin on of the Volume Pot. Then take the ground wire and connect it to the negative power rail.
Turn on / connect the battery (power supply)—generally you want to do this first. Then connect your phone and play some music. You should immediately hear something. Turn your phones volume up about half way, then use the gain and volume pots to adjust the signal until it sounds loud and clean.
My circuit is a modified version of a circuit by the good people at circuitbasics.com. Although I have made changes the basics are from their design.